1. Field of the Invention
An aspect of the present invention relates to a resonator and a filter used in a microwave device, such as a broadcasting device, a communications device, a measuring device.
2. Description of the Related Art
As the simplest resonator structure using a strip line or a microstrip line, there is known a structure consisting of: a conductor line having a half wavelength (or a multiple thereof) at a resonance frequency; a dielectric substrate; and a ground plane. When the resonator resonates with a mode in which a current flow along the conductor line, a current density in the resonant state is most concentrated at an edge of the conductor line, and the concentration tendency becomes more noticeable with an increase in frequency.
When the above-mentioned structure is adapted to a microwave resonator for a high-power signal, such as a signal having a power of 1 W or more, a current concentration on the edge poses a problem. Because, a particularly-large current density is induced at an edge of the conductor line by the high power signal, and a conductor loss arising in the edge consequently becomes a dominant cause for a loss in the resonator. Further, when a current density exceeds an allowable level for the conductor material, the conductive property of the conductor material may be destroyed. For example, when a superconducting material is used for the conductor line, an excess current density at the edge may destroy the conductive property of the conductor line.
A method for relaxing the current concentration at the edge of the straight-type conductor line by forming a plurality of slits at uniform intervals therealong is proposed, in JP-H08-321706-A. A method which is an improvement upon the method proposed in JP-H08-321706-A and which is proposed in JP-H11-177310-A is a method for forming a single slit or a plurality of slits, along a straight-shaped conductor line, in only an edge thereof.
The simplest shape of the conductor line is a straight line shape. In addition, to be mounted in a limited space, the conductor line may be formed to have a bent portion. For example, a hairpin shape, a spiral shape, a meandering shape, an L shape, an M shape, and an S shape have been proposed.
When a transmission line, such as a strip line or a microstrip line, formed in a straight shape is used as a resonator, the method of JP-H08-321706-A or JP-H11-177310-A may be effective. However, when a bent shape is applied to a conductor line, a current concentration arises at an inner-side edge of the bent portion.